1. Field of the Invention
The present invention relates to an EL (electroluminescent) display apparatus which is used as a display apparatus for various information terminal devices, inboard aircraft displays, inboard vehicle displays, etc., and more particularly to a transparent thin-film EL display apparatus enabling visual recognition of a background therethrough.
2. Related Arts
An EL display apparatus is a type of display apparatus which utilizes the phenomenon that luminescence occurs in a luminescent substance, based on zinc sulfide (ZnS) or the like as a host material, when an electric field is applied thereto. This type of display unit has drawn attention as a self-luminescence type planar display apparatus. Typically, an EL element obtained by sequentially laminating a first electrode, a first insulation layer, a luminescent layer, a second insulation layer, and a second electrode in the order mentioned is formed on a retaining substrate and this EL element is hermetically sealed by a protective substrate. Further, the color of the luminescence emitted therefrom can be variously changed according to the type of luminescence center element added to the luminescent layer. For example, the EL element emits an amber color by adding manganese (Mn) to the luminescent layer with ZnS used as the host material thereof, and emits green, red, blue, and white colors by respectively adding terbium fluoride (TbF.sub.3), samarium chloride (SmCl.sub.3), thulium chloride (TmCl.sub.3), and praseodymium fluoride (PrF.sub.3).
Among the EL display apparatuses constructed as described above, a transparent thin-film EL display apparatus in which the retaining substrate, EL element, and protecting substrate are each composed of transparent material is disclosed in "Nikkei Electronics" (p. 98, FIG. 9), published Aug. 16, 1982. By mounting such a transparent thin-film EL display apparatus on the dashboard of an automobile, it was expected that using it as a transmission type display apparatus would enable the driver to visually recognize displayed information such as speed etc. while directing his line of sight toward the front of the automobile to drive, and in addition would not become an obstacle to the driver's forward field of vision. In view of this, in order to enhance forward visibility, a technique has been proposed to provide an antireflection film on the protective substrate (transparent rear substrate) located on the rear of the transparent thin-film EL display apparatus so as to enable clear visibility of an object to be observed located at the rear of the transparent thin-film EL display apparatus, i.e. located at the front through this display apparatus.
However, in a case where the EL display apparatus is used as a heads-up display in a vehicle as mentioned above, early morning sunlight or late afternoon sunlight, when the altitude of the sun is relatively low, may enter from the rear side of the display apparatus. In addition, where light is incident through the rear side of the transparent thin-film EL display apparatus, there is the problem that the displayed information on the EL display apparatus is hard to see. For this reason, it is necessary for the visibility of the front view viewed through the EL display apparatus and the clarity of the display to be compatible.
Enhancing the contrast of the display has been considered as one technique for improving the clarity of the display. In order to enhance the contrast of the display of the transparent thin-film EL display apparatus, methods such as adhering a black vinyl sheet or the like onto the transparent rear plane substrate or applying a color coating material to the substrate are known. Further, in Japanese Examined Patent Publication No. 58-55634, a technique of including a color dye in a protective injection fluid of the thin-film EL element has been proposed.
However, the above-mentioned techniques for enhancing the contrast of the display sacrifice transparency, which is the most significant feature of the transparent thin-film EL display apparatus. Namely, the use of such techniques results in the deterioration of forward visibility through the EL display apparatus.